Excited state properties of formamide in water solution: an ab initio study.

We present ab initio quantum calculation of the optical properties of formamide in vapor phase and in water solution. We employ time dependent density functional theory for the isolated molecule and many-body perturbation theory methods for the system in solution. An average over several molecular dynamics snapshots is performed to take into account the disorder of the liquid. We find that the excited state properties of the gas-phase formamide are strongly modified by the presence of the water solvent: the geometry of the molecule is distorted and the electronic and optical properties are severely modified. The important interaction among the formamide and the water molecules forces us to use fully quantum methods for the calculation of the excited state properties of this system. The excitonic wave function is localized both on the solute and on part of the solvent.

Proton quantum coherence observed in water confined in silica nanopores.

Deep inelastic neutron scattering measurements of water confined in nanoporous xerogel powders, with average pore diameters of 24 and 82 A, have been carried out for pore fillings ranging from 76% to nearly full coverage. DINS measurements provide direct information on the momentum distribution n(p) of protons, probing the local structure of the molecular system. The observed scattering is interpreted within the framework of the impulse approximation and the longitudinal momentum distribution determined using a model independent approach. The results show that the proton momentum distribution is highly non-Gaussian. A bimodal distribution appears in the 24 A pore, indicating coherent motion of the proton over distances d of approximately 0.3 A. The proton mean kinetic energy W of the confined water molecule is determined from the second moment of n(p). The W values, higher than in bulk water, are ascribed to changes of the proton dynamics induced by the interaction between interfacial water and the confining surface.

Ab initio calculation of optical spectra of liquids: many-body effects in the electronic excitations of water.

We present ab initio calculations of the excited state properties of liquid water in the framework of many-body Green's function formalism. Snapshots taken from molecular dynamics simulations are used as input geometries to calculate electronic and optical spectra, and the results are averaged over the different configurations. The optical absorption spectra with the inclusion of excitonic effects are calculated by solving the Bethe-Salpeter equation. The insensitivity of screening effects to a particular configuration make these calculations feasible. The resulting spectra, which are strongly modified by many-body effects, are in good agreement with experiments.